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Old 11-24-2009, 12:27 AM   #24 (permalink)
brtmac
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Join Date: May 2008
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The power required to move an object through a fluid (air is considered a fluid) increases as the cube of the velocity ( http://en.wikipedia.org/wiki/Drag_(physics) ).

The blades have drag, even when not providing lift (parasitic drag). This drag increases as the cube of the speed of the blade. So, as you increase rotor speed you cube your power consumption, even while sitting on the ground generating no lift.

Increased rotor speed gives increased ability to generate lift because you get more lift with the same pitch. And, you can only go so low on rotor speed because at some point the pitch necessary to hover will be too close to the stall pitch of the blade and you won't be able to generate enough lift to do anything useful.

So, it's all give and take. High head speeds for crazy power, but low run times. Low headspeeds for long runtimes, but less insane power. The best thing to do is find the lowest speed that gives you the power you want. This is where the overall weight of the heli comes in. The lower the weight, the less power you need to do what you want. The less power you need, the lower the headspeed you can run. The lower the headspeed, the less parasitic power drain you have (power needed to just spin the rotor, not generate lift) and the longer runtime you get.

Another way to think about is that a given motor/esc/battery setup has a maximum useful power output. The more of that you use to shove the blades through the air, the less you have available for actually generating lift. What does happen is that when you increase pitch to generate lift, and you hit the peak output of your motor/esc/battery combo, you have to start barrowing from the headspeed bank and you bog. It's quite likely that for a lot of the headspeed crazy setups out there you'd actually have *more* real power to generate lift by lowering the headspeed.
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